Diuretics are drugs which increase the flow of urine. Considered in a broader sense, however, these agents augment the secretion and modify the character of the urine:

1. By increasing the amount.

2. By rendering the urine acid.

3. By rendering the urine alkaline.

4. By removing waste products or increasing the solid constituents of the urine.

By preventing the decomposition of the urine.

The last-named action is peculiar to benzoic * and salicylic * acids, cubeb, copaiba, uva ursi, oil of sandalwood, volatile oils, * saccharin, hexamethylenamina, and salol.

The following medicines affecting the urinary system are called Lithontriptics, because of their supposed power to prevent the formation of concretions in the urinary passages or to dissolve them when formed:

Piperazin, potassium salts,* lithium salts,* ammonium benzoate,* benzoic acid,* dilute nitric acid.*

Among the principal drugs which render the urine acid are -benzoic * and salicylic acids,* and many of their salts, immoderate amounts of the vegetable acids,* and sour wines.*

The alkalies,* particularly the potassium and lithium salts, when taken internally, render the urine alkaline in reaction.

Diuretics may be either direct or indirect - i. e., they may act on the kidneys themselves or upon certain structures outside the kidneys. The structures in the kidneys which have to do with the elimination of water, solids, etc., are: 1. The Malpighian tufts, which eliminate principally water, but also mineral salts and certain pathological and foreign substances which may be present. 2. The glandular epithelium lining the convoluted tubules, which excretes waste-products, such as urea, etc. 3. The constricted portion of the tubules, serving to prevent (according to the Cohnheim-Bauman theory) the too-rapid escape of water, thus allowing time for its absorption in cases where it is desirable that the water be retained in the system.

The functional activity of these various structures is regulated by the nervous mechanism through its influence upon the blood-supply. For example, the supply of blood to the glomeruli is influenced.

The drugs marked with an asterisk (*) are described elsewhere in the present work. 522 largely by the size of the blood-vessels, regulated by the vasoconstrictor and vasodilator nerves ; but it has not been proved that the secretory cells are in any way affected by the nervous mechanism. Diuretics act:

1. By increasing the general blood-pressure.

2. By causing local dilatation of the renal arterioles.

3. By directly stimulating the glandular secreting renal structures.

4. By simple mechanical force.

The following table, modified from Brunton's work on Pharmacology, Therapeutics, and Materia Medica, serves to elucidate the methods by which the various diuretic agents probably exert their influence:

The secretion of urine is considerably influenced by the activity of the skin and bowels; for instance, when the cutaneous glands are stimulated and there is free perspiration, a diminished urinary secretion ensues. The functional activity of the skin and sudoriparous glands depends greatly upon the amount of blood supplied to them. Whatever augments the flow of blood to these structures increases the secretion of the sweat-glands. Consequently, external warmth dilates the cutaneous blood-vessels and promotes diaphoresis, while cold contracts the cutaneous vessels, diverting the flow of blood to the internal organs, thereby increasing the secretion from the kidneys and lessening that from the skin.

It will be seen, therefore, that the functions of the skin and kidneys are compensatory, the compensation being also partially observable in the mutual relations between the bowels and kidneys. It is well known that when there is active purgation, with frequent watery movements from the bowels, the amount of urine secreted is proportionally diminished.

Any drug which increases the general blood-pressure and forces a larger blood-supply into the kidneys augments the pressure in the glomeruli, distending the capsule and enlarging the area of the osmotic membrane, which action, combined with an increase in the circulation, promotes and facilitates filtration, thereby augmenting the amount of urine.

The blood-pressure in the glomeruli, as has been said, may be increased by additional pressure in the general circulation. It may be raised also locally through dilatation of the afferent blood-vessels supplying the Malpighian corpuscle, or contraction of the efferent vessels, allowing a smaller quantity of blood to escape from the glomerules.

The secreting structures of the convoluted tubules are stimulated by the influence of certain drugs which are carried in the blood, acting as excitants upon the secreting cells. This necessarily requires an extra supply of blood to the part furnishing material for the extra secretion.

The imbibition of large amounts of water, while increasing the blood-pressure to some extent, mechanically increases the amount of water eliminated by the kidney. This is commonly known as the " flushing action," and renders the urine more dilute.

In congested conditions of the kidneys certain remedial measures - such as local venesection, dry cupping, warm fomentations, etc. - promote renal secretion.

Therapeutics. - 1. To remove excessive accumulation of fluid in the tissues and serous cavities of the body when the blood-pressure is low.

For this purpose the most efficient service is derived from the use of drugs which act by increasing the systemic blood-pressure, and stimulating the secreting cells of the kidneys.

Ordinarily, the agents most beneficial in cardiac dropsy or dropsies due to venous congestion are digitalis, calomel, scoparius, squill, diuretin, etc.

2. To remove excess of fluid from the body when the blood-pressure is about normal, as in cases of hepatic cirrhosis with dropsy.